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Adsorption and Catalysis

Adsorption versus Absorption

Adsorption Absorption

H H H H H H H H H

H2 adsorption onpalladium

H2 absorption palladium hydride

Surface process bulk process

Nomenclature

Substrate or adsorbent: surface onto which adsorption can occur.example: catalyst surface, activated carbon, alumina

Adsorbate: molecules or atoms that adsorb onto the substrate.example: nitrogen, hydrogen, carbon monoxide, water

Adsorption: the process by which a molecule or atom adsorb onto a surface of substrate.Coverage: a measure of the extent of adsorption of a specie onto a surface

Exposure: a measure of the amount of gas the surface had been exposed to ( 1 Langmuir = 10-6 torr s)

H H H H H H H H H H H H H Hadsorbate

adsorbent

coverage fraction of surface sites occupied

Types of Adsorption Modes

Physical adsorption orphysisorption

Chemical adsorption orchemisorption

Bonding between molecules andsurface is by weak van der Waalsforces.

Chemical bond is formed betweenmolecules and surface.

Characteristics of Chemi- and Physisorptions

Chemisorption

virtually unlimited range

wide range (40-800 kJmol-1)

marked difference forbetween crystal planes

often dissociative andirreversible in many cases

limited to a monolayer

activated process

Physisorption

near or below Tbp of adsorbate(Xe < 100 K, CO2 < 200 K)

heat of liquifaction (5-40 kJmol-1)

independent of surface geometry

non-dissociative andreversible

multilayer occurs often

fast, non-activated process

Properties

Adsorption temperature

Adsorption enthalpy

Crystallographicspecificity

Nature of adsorption

Saturation

Adsorption kinetic

Analytical Methods for Establishing Surface BondsInfrared Spectroscopy

Atoms vibrates in the I.R. range

• chemical analysis (molecular fingerprinting)• structural information• electronic information (optical conductivity)

IR units: wavenumbers (cm-1),10 micron wavelength = 1000 cm-1

Near-IR: 4000 – 14000 cm-1Mid-IR: 500 – 4000 cm-1Far-IR: 5 – 500 cm-1

http://infrared.als.lbl.gov/FTIRinfo.html

I.R. Measurement

I.R. Spectrum of CO2

Symmetric Stretch

Assymmetric Stretch

Bending mode

A dipole moment = charge imbalance in the molecule

I.R. Spectrum of NO on Pt

Adsorption decreases

Molecular conformationchanges

I.R. Spectrum of HCN on Pt

0.15 L HCN, 100 Kweak chemisorption

1.5 L HCN, 100 Kphysisorption

30 L HCN, 200 Kdissociative chemisorption

(H-CN)(HCN)(HCN)

(a) (b) (c)

Adsorption Rate

Rads = k C x

x - kinetic order k - rate constantC - gas phase concentration

Rads = k’ P x

x - kinetic order k’ - rate constantP - partial pressure of molecule

Rads = A C x exp (-Ea/RT)

Activation energyFrequency factor

Temperature dependencyof adsorption processes

Molecular level event

Adsorption Rate

Rads = S • F = f() P/(2mkT)0.5 exp(-Ea/RT)

Sticking coefficient

S = f() exp(-Ea/RT)

where 0 < S < 1

Flux (Hertz-

Knudsen)

F = P/(2mkT)0.5

P = gas pressure (N m-2)m = mass of one molecule (Kg)T = temperature (K)

(molecules m-2 s-1)

Note: f() is a function of surface coverage special case of Langmuir adsorption f() = 1-

E(), the activation energy is also affected by surface coverage

Sticking Coefficient

S = f() exp(-Ea/RT)where 0 < S < 1

S also depends on crystal planes and may be influenced by surface reconstruction.

Tungsten

Sticking Coefficient

Sticking CoefficientSteering Effects

Surface Coverage ()

Estimation based on gas exposure

Rads = dNads/dt = S • F

Nads S • F • tExposure time

Molecules adsorbed perunit surface area

Nearly independentof coverage for mostsituations

Adsorption Energetics

surface

adsorbate

Potential energy (E) for adsorption is only dependent on distancebetween molecule and surface

P.E. is assumed to be independent of:• angular orientation of molecule• changes in internal bond angles and lengths• position of the molecule along the surface

Physisorption versus chemisorption

Adsorption Energetics

surface

E(ads) E(ads) < E(ads)Physisorption Chemisorption

small minima large minimaweak Van der Waal formation of surfaceattraction force chemical bonds

repulsive force

attractive forces

Chemisorption

Physical Adsorption

metal surface

nitrogen

Van der Waal forces

E(d)0.3 nm

Note: there is no activation barrier for physisorption fast process

Applications:• surface area measurement• pore size and volume determination• pore size distribution

The Brunauer-Emmett-Teller Isotherm

BET isotherm

where: n is the amount of gas adsorbed at P nm is the amount of gas in a monolayer P0 is the saturation pressure n at P = P0

C is a constant defined as:

H1 and HL are the adsorption enthalpy of first and subsequent layers

BET Isotherm

Assumptions• adsorption takes place on the lattice and molecules stay put,• first monolayer is adsorbed onto the solid surface and each layers can start before another is finished,• except for the first layer, a molecule can be adsorbed on a given site in a layer (n) if the same site also exists in (n-1) layer,• at saturation pressure (P0), the number of adsorbed layers is infinite (i.e., condensation), • except for the first layer, the adsorption enthalpy (HL) is identical for each layers.

Activated Carbon

Surface area ~ 1000 m2/g

Surface Area Determination

BET surface area by N2 physisorption

- adsorption- desorption

Plot P/n(P0-P) versus P/P0

calculate c and nm from the slope (c-1/ nmc) andintercept (1/nmc) of the isothermmeasurements usually obtained for P/P0 < 0.2

c = 69.25nm = 4.2 x 10-3 molArea = 511 m2/g

c = 87.09nm = 3.9 x 10-3 molArea = 480 m2/g

BET Measurements

DegassingDegassing

Pure gas introduces into supply Pure gas introduces into supply chamber chamber constant P constant P11 T T11 are are

recorded recorded V V11

Gas flows into adsorption cellGas flows into adsorption cell

PP22 and T and T22 are recorded when are recorded when

equilibrium is reached equilibrium is reached V V22

P 1 T 1 P 2 T 2

To vacuum

Gas cylinder

Gas SupplyChamber

AdsorptionCell

Volumetric Method

BET Measurements

Dynamic Method

DegassingDegassingFlow carrier gas (He)Flow carrier gas (He)

Pulse NPulse N22/He into adsorption cell /He into adsorption cell at a given Pat a given PN2N2

Record the amount of nitrogen Record the amount of nitrogen adsorbed using TCD adsorbed using TCD Calculate surface areaCalculate surface area

(Rouquerol, 1999)

BET Measurements

Gravimetric Method

DegassingDegassingRecord initial weight of adsorbent Record initial weight of adsorbent MM11

Introduce pure gas into Introduce pure gas into adsorption celladsorption cellRecord the adsorbent equilibrium Record the adsorbent equilibrium weight Mweight M22

Record the equilibrium pressureRecord the equilibrium pressure

(Rouquerol, 1999)

Adsorption Isotherm

Adsorption Isotherm:Adsorption Isotherm:– The equilibrium relationship between the amount adsorbed The equilibrium relationship between the amount adsorbed

and the pressure or concentration at constant temperature and the pressure or concentration at constant temperature (Rouquerol et al., 1999).(Rouquerol et al., 1999).

Importance of ClassificationImportance of Classification– Providing an efficient and systematic way for theoretical Providing an efficient and systematic way for theoretical

modeling of adsorption and adsorbent characteristics modeling of adsorption and adsorbent characteristics determinationdetermination

Rouqerol, F., J., Rouquerol and K., Sing, Adsorption by Powders and Porous Solids: Principles, Methodology and Applications, Academic Press, London (1999).

Adsorption Isotherm

IUPAC Classification

Adsorption Isotherm

Type I(Activated Carbon,

Zeolites)

Micropores

(< 2 nm)

Type III(Bromine onsilica gel)*

Type V(Water oncharcoal)*

Weakinteraction

Type II(Clay, pigments,

cements)

Type IV(oxide gels,

zeolites)

Stronginteraction

Macropores

(> 50 nm)

Mesopores

(2 – 50 nm)

* Do, D. D., Adsorption Analysis: Equilibria and Kinetics, Imperial College Press, London (1998).

Adsorption Isotherm

Capillary Condensation

Mesopores Mesopores

Capillary condensationCapillary condensation

Hysteresis occursHysteresis occurs

Different hysteresis Different hysteresis Different network structure Different network structure

Narrow distribution of uniform pores Narrow distribution of uniform pores Type IVa Type IVa

Complex structure made up of interconnected networks of Complex structure made up of interconnected networks of different pore sizes and shapes different pore sizes and shapes Type IVb Type IVb

Adsorption Isotherm

Type VI Isotherm

Highly uniform surfaceHighly uniform surface

Layer by layer adsorptionLayer by layer adsorption

Stepped isothermStepped isotherm

Example:Example:

Adsorption of simple non-porous Adsorption of simple non-porous molecules on uniform surfaces molecules on uniform surfaces (e.g. basal plane of graphite)(e.g. basal plane of graphite)

Adsorption Isotherm

Composite Isotherm

N2 adsorption in (a) micropores and (c) micropores and mesopores

Type I Type I & IV

(Rouquerol, 1999)

Chemical Adsorption

Pt surface

Note: there is no activation barrier for adsorption fast process, there us an activation barrier for desorption slow process.

Applications:• active surface area measurements• surface site energetics• catalytic site determination

= strength of surface bonding

= equilibrium bond distance

= H(ads)

Ea(ads) = 0

Ea(des) = - H(ads)

Chemical Adsorption ProcessesPhysisorption + molecular chemisorption

E(d) physisorption

chemisorption

Chemical Adsorption ProcessesPhysisorption + dissociative chemisorption

E(d)dissociation

chemisorption

H2H2 2 H

physisorption

atomic chemisorption

Note: this is an energy prohibitive process

physisorption/desorption chemisorption

physisorption

direct chemisorption

physisorption

Chemical Adsorption ProcessesEnergy barrier

Ea(ads) ~ 0

Ea(ads) > 0

Chemical Adsorption ProcessesEnergy barrier

~ -H(ads)

- Eades = -E(ads)

Chemical Adsorption is usuallyan energy activated process.

Formation of Ordered Adlayer

Ea(surface diffusion) < kT

activated carbon CH4

Krypton

Formation of Ordered Adlayer

Chlorine on chromium surface

Adsorbate Geometries on Metals

Hydrogen and halogens

Hydrogen

1-H atom per 1-metal atom

2-D atomic gas

Halogens high electronegativity dissociative chemisorption

Halogen atom tend to occupy high co-ordinationsites:

ionic bonding

(111) (100)

compound

Oxygen and Nitrogen

(111) (100)

Oxygen

both molecular and dissociativechemisorption occurs.molecular chemisorption -donor or-acceptor interactions.

dissociative chemisorption occupy highest co-ordinated surface sites, alsocauses surface distorsion.

Nitrogen

molecular chemisorption -donor or-acceptor interactions.

Carbon monoxide

forms metal carbides with metals locatedat the left-hand side of the periodic table.

molecular chemisorption occurs on d-blockmetals (e.g., Cu, Ag) and transition metals

CO COTerminal (Linear)

all surface

Bridging (2f site)

all surface

Bridging (3f hollow)

(111) surface

C metal carbide

Ammonia and unsaturated hydrocarbons

Ammonia

NH2 (ads) + H (ads) NH (ads) + 2 H (ads) N (ads) + 3 H (ads)

Ethene

2HC=CH2

Active Surface Area Measurement

ost common chemisorption gases: hydrogen, oxygen and carbon monoxide

Pulse H2, O2

or CO gases

exhaustcarrier gashelium or argon

thermal conductivity cell (TCD)

furnace

catalyst

Catalyst Surface Area and Dispersion Calculation

Pulse H2 thentitrate with O2

exhaustcarrier gashelium or argon

thermal conductivity cell (TCD)

furnace

1 g 0.10 wt. % Pt/-Al2O3

T = 423 K, P = 1 bar(STP)

3.75 peaks (H2)4.50 peaks (O2)

Avogrado’s number: 6.022 x 1023

Pt lattice constant: a = 3.92 (FCC) Calculate surface area of Pt and its dispersion.

Isotherms

Langmuir isotherm

S - * + A(g) S-A

surface sites

Adsorbed molecules

H(ads) is independent of the process is reversible and is at equilibrium

[S-M] [S - *] [A]

S-M] is proportional to [S-*] is proportional to 1-[A] is proportional to partial pressure of A

Isotherms

Langmuir isotherm

(1-) P

Where b depends only on the temperature

bP 1+ bP

Molecular chemisorption

Where b depends only on the temperature

(bP)0.5

1+ (bP)0.5 =

Dissociative chemisorption

Variation of as function of T and P

bP at low pressure 1 at high pressure

0 0.2 0.4 0.6 0.8 1

b when T b when H(ads)

Determination of H(ads)

0 0.2 0.4 0.6 0.8 1

(P1, T1) (P2, T2)

InP( ads

R1/T ) =const

Adsorption Isotherms

Henry’s Adsorption Isotherm

Special case of Langmuir isotherm

bP << 1

= bPV = k’P where k’ = bV

The Freundlich Isotherm

Adsorption sites are distributed exponentially with H(ads)

H(ads)

(1-i)biP =

iNi Ni

In = InP + B

kP1/n =Valid for low partial pressuremost frequently used for describing pollutant adsorption on activated carbons

The Temkin Isotherm

H(ads) decreases with

A InBP = H(ads)

Valid at low to medium coveragegas chemisorption on clean metal surfaces

Thermal Desorption Spectroscopy

Thermal desorption spectra of CO on Pd(100) after successive exposure to CO gases

0.2 - 50 L

Chemical Adsorption

Pt surface

Note: there is no activation barrier for adsorption fast process, there us an activation barrier for desorption slow process.

Applications:• active surface area measurements• surface site energetics• catalytic site determination

= strength of surface bonding

= equilibrium bond distance

= H(ads)

Ea(ads) = 0

Ea(des) = - H(ads)

Thermal desorption spectra of CO on Pd(100) after successive exposure to CO gases

Desorption Rate

{-dNadT

dTdt } = Na

mk exp( -Ed

Linear heating rate

T = T0 + t

Assuming k and Ed are independent of coverageand m = 1 (i.e., first order desorption)

0.2 - 50 L-dNa

d -dNadTdT

RTp2 = exp( -Ed

RT ) k

Determination of Edes using different heating rates ()

RTp2 = exp( -Ed

RT ) k

slope, m EaTPD provides important informationon adsorption/desorption energeticsand adsorbate-surface interactions.

Thermal desorption spectra of CO on Ni(100) after successive exposure to CO gases

0.2 - 50 L Assuming k and Ed are independent of coverageand m = 2 (i.e., first order desorption)-dNa

d -dNadTdT

[ ] Second order desorption

RTp2 = exp( -Ed

RT ) k

2(Na)p

Characterized by a shift in the peak maximatoward lower temperature as the coverageincreases

Activation Energies for CO Desorption

Influence of Surface Overlayer

Catalyst poison, strong adsorbates and coke

Sulfur-treatedcatalyst

Clean catalystCO desorption

Ordered Adsorbate layer

H2/Rh(110) O2/Rh(110)

TPD from Rh(110)

Ordered Adsorbate layer

benzene/ZnO(1010)

Kelvin Probe

Measures the change in work function ()

Typical Kelvin probe for adsorption studies

Scanning Kelvin probe for surface work function (i.e., elemental and compositional) imaging

also known as scanning electricalfield microscopy

Kelvin Probe

Basic principle

Vibrating capacitor measures is the least amount of energy needed for an electron to escape from metal to vacuum.is sensitive optical, electrical and mechanical properties of materials

refref

Dr. King Lun YeungDr. King Lun Yeung

Department of Chemical EngineeringDepartment of Chemical Engineering

Hong Kong University of Science and Hong Kong University of Science and TechnologyTechnology

CENG 511 Lecture 3

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